Rectangular batteries using a nonaqueous electrolyte have a configuration in which a battery case made of, e.g., a metal houses an electrode group and an electrolyte. The electrode group is formed by winding a positive electrode and a negative electrode with a separator interposed therebetween. The separator electrically insulates the positive electrode and the negative electrode from each other, and holds the electrolyte. The battery case has an opening at one end, and the opening is sealed by a sealing plate having the substantially same shape as the opening after the electrode group is housed in the battery case.
In many cases, the sealing plate is connected to the battery case, and in that case, the sealing plate and the battery case have the same polarity. In other words, when a positive electrode lead is connected to the sealing plate, the battery case connected to the sealing plate serves as an external terminal of the positive electrode. A through hole is formed in the sealing plate, and a terminal rivet is inserted into the through hole through a gasket. The terminal rivet is connected to a current collector plate disposed under the sealing plate through an insulating plate, and a negative electrode lead is connected to the current collector plate, whereby the terminal rivet serves as an external terminal of the negative electrode. In this way, the terminal of the positive electrode and the terminal of the negative electrode are drawn to the outside the battery.
In such a rectangular battery, the battery case is substantially rectangular in transverse cross section. Therefore, if a predetermined pressure is applied from the outside of the battery, the battery is more likely to be deformed when the pressure is applied along a longer side direction in the transverse cross section of the battery case than when the pressure is applied along a shorter side direction in the transverse cross section of the battery case. Therefore, if a large pressure that deforms the rectangular battery is applied from the outside of the battery, the electrode group is significantly deformed at both ends in the longer side direction thereof to damage a part of the separator. Due to the damage, a positive electrode active material and a negative electrode active material are locally in contact with each other to cause an internal short-circuit. When the internal short-circuit occurs, a large short-circuit current intensively flows between the positive electrode active material and the negative electrode active material which are in contact with each other, and Joule heat is generated. Therefore, the battery may be abnormally overheated.
In order to address such a problem, for example, Document 1 shows a battery in which a cap assembly sealing an opening of the battery case is provided with a terminal plate having a polarity different from that of the battery case. Such a battery, if the battery case is crushed along its longer side direction, allows the battery case and the terminal plate to be in contact with each other in an initial state to immediately discharge the battery, whereby abnormal overheat of the battery can be prevented.